The invention relates to techniques for contactless communication between a portable object and a terminal.
Contactless data exchange is well known; applications of this technique include, in a non-limiting manner, controlling access, electronic payment (xe2x80x9celectronic pursexe2x80x9d type applications), and remote payment, e.g., for access to and payment of public transport.
In the latter example, each user is provided with a portable object of the xe2x80x9ccontactless cardxe2x80x9d or xe2x80x9ccontactless badgexe2x80x9d type, which object is suitable for exchange information with a fixed (or possibly moving) terminal by bringing the badge close to the terminal so as to enable non-metallic mutual coupling to take place therebetween (the term xe2x80x9cterminalxe2x80x9d is used in the present description to designate the data transmitter/receiver apparatus suitable for co-operating with the portable objects).
More precisely, the coupling is performed by varying a magnetic field produced by an induction coil (the technique known as the xe2x80x9cinduction methodxe2x80x9d). To this end, the terminal includes an inductive circuit excited by an alternating signal which produces an alternating-magnetic field in the surrounding space. When in said space, the portable object detects the field and responds by modulating the load constituted by the portable object coupled to the terminal; this variation is detected by the terminal, thereby establishing the looked-for both-way communication.
The invention relates to the particular case of a portable object that is a remotely-powered portable object, i.e., it takes its power from the magnetic energy emitted by the terminal, and more precisely the invention relates to the case where the remote power is picked up by the portable object using the same coil as that which is used for the communication function.
The invention also relates to the case where information is transmitted from the terminal to the portable object by amplitude modulation; in which case the portable object includes means for demodulating the amplitude of the signal picked up by the coil.
U.S. Pat. No. 4,650,981 describes a contactless communications system of that type in which the portable object is placed in the air gap of a magnetic circuit of the terminal, coupling being achieved when the user inserts the portable object in a read slot of the terminal. The coil of the portable object is thus placed in the magnetic circuit of the terminal, thereby ensuring the looked-for coupling, with information being transmitted in both directions between the terminal and the portable object, and with the portable object being remotely powered from the magnetic energy produced by the terminal. To this end, the portable object includes a single coil that picks up the magnetic field from the terminal and that is associated with converter means (for rectifying and filtering) to enable a DC power supply voltage to be produced, and also with amplitude demodulator means operating downstream from the converter means in order to extract the information content from the signal radiated by the terminal.
One of the objects of the present invention is to provide a contactless data exchange technique of the above-mentioned type, but which is usable with a magnetic field that is radiated into free space, i.e., when the portable object is merely present in a predetermined volume around the coil of the terminal, at an arbitrary orientation and at a distance from the coil that can vary: the purpose is to be able to establish contactless communication with xe2x80x9chands-freexe2x80x9d type terminals (e.g. when passing through an inspection gate) or with terminals where the user is asked merely to bring the badge into a read zone of small size or to place the badge against such a zone, but at any orientation and with a certain amount of geometrical latitude between the terminal and the portable object.
Under such circumstances, remote powering suffers from the drawback of depending on the surrounding magnetic field, which can vary very greatly given the close or remote proximity of the portable object to the terminal. Such very large variations in magnetic field, once rectified and filtered, give rise to very large variations in the power supply voltage for the portable object, which variations must be eliminated by means of a suitable stabilizer stage.
Another drawback of remote powering lies in possible interference between variations in the power supply voltage (due to variations in the mean level of the magnetic field as a function of the distance between the object and the terminal or due to erratic variations in the amount of current consumed) and the modulation of the magnetic field, when said modulation is amplitude modulation: interfering or minor variations can thus be wrongly interpreted as signal modulation, with the consequence of introducing transmission errors.
Conversely, when the object is transmitting signals to the terminal, modulation by varying the load of the tuned circuit implies forced variation in the amount of current consumed by the portable object, and that has repercussions on the general power supply to the circuits of the object, with the danger of the object being under-powered during certain stages of the modulation.
As shown above, remotely powering a portable object by a magnetic field is not without drawbacks (these aspects are explained in the detailed description), and until now that has limited the use of this technique in spite of its advantages, or has restricted it to highly specific applications, for example when the distance between the terminal and the object is small and constant, as is the case in above-mentioned U.S. Pat No. 4 650 981, which is generally not the case of remote payment applications.
An object of the invention is to remedy those various drawbacks by proposing a contactless communications system between a terminal and a remotely powered portable object, enabling the electronic circuits of the object to be powered in complete safety, without any risk of interference, and with optimum management of the power received from the terminal and picked up by the portable object.
Another object of the invention is to be able to allow synchronous type communication between the terminal and the portable object, i.e., communication in which the operation of the card is clocked by a clock signal defined by the terminal. This technique is commonly used with cards that have contacts (ISO standard 7816-3 specifically providing a series of contacts for transmitting the clock signal in the event of synchronous communication), however, in spite of its clear advantages, this technique is not often used in contactless systems because of the difficulties involved in transmitting clock signal information from the terminal to the portable object.
The system of the invention is of the above-mentioned general type taught by above-mentioned U.S. Pat. No. 4,650,981, it is a system in which the terminal includes: a coil suitable for emitting a magnetic field; data transmission means co-operating with the coil and comprising alternating signal generator means and amplitude modulator means; and data reception means co-operating with the coil. The portable object includes an electric circuit that is remotely powered by the terminal, and comprising: a coil for picking up said modulated magnetic field coming from the terminal or for producing a response by modulated disturbance of the magnetic field; converter means co-operating with the coil of the portable object to transform the magnetic field picked up thereby into a DC voltage for powering the circuit of the object, said means comprising a rectifier stage and a filter stage; and data transmission means and data reception means also co-operating with the coil of the portable object, the data reception means including means for demodulating the amplitude of the signal picked up by the coil, said amplitude demodulator means operating on the signal delivered at the output from the rectifier and filter stages.
The system is characterized in that each of said coils forms a portion of a tuned resonant circuit radiating the field in empty space; and the amplitude modulation of the magnetic field emitted by the terminal is modulation at low depth, with a modulation ratio that is typically less than or equal to 50%.
The invention also relates to the portable object and to the terminal of the above-specified system, considered as independent characteristic entities.
According to various advantageous subsidiary characteristics of the invention:
the modulation ratio is less than 20%;
the amplitude modulator means are variable threshold means comparing the instantaneous value of the signal applied to the input thereof with a mean value of the same signal;
the amplitude demodulator means are means sensitive to the rate at which the instantaneous value of the applied signal increases;
the converter means further comprise, downstream from the rectifier and filter stages, a stabilizer stage including a shunt regulator element mounted in parallel with the circuit to be powered between its power supply terminals and associated with a resistive component connected in series in the power supply line of the circuit, the shunt regulator element taking off a variable fraction of the power supply current for the circuit so that the resistive element and the shunt regulator element dissipate any excess power that is not required for operation of the circuit, in such a manner that, correspondingly, the power supply voltage at the terminals of the circuit is stabilized, the voltage excursion at the terminals of the upstream tuned element is limited, and variations in current consumption are prevented from having an influence upstream on the amplitude of the signal to be demodulated;
the portable object comprises means for selectively and temporarily inhibiting operation of the shunt regulator; in which case means are preferably provided for detecting the type of communication, contactless or via contacts, and in which the selective and temporary inhibition of the operation of the shunt regulator is implemented in response to detecting communication of a type that passes via contacts;
the entire electronic circuit with the exception of the coil of the tuned element is implemented in integrated monolithic technology; and
the data transmitter means are means that operate by modulating current consumption downstream from the tuned circuit, and the circuit is capable of operating in two power consumption modes, with nominal power consumption and with low power consumption, means being provided for putting the circuit into low power consumption mode before the data transmission means begin to perform said modulation.